Specified using the optional 'omega' entry (Function1 type), e.g. for a constant
value:

      omega           12.56;

Note that the swirl contribution is applied in addition to the velocity set by
the 'flowType' option. For example, for the 'constantVelocity' option, parcels
are initially set the velocity according to the UMag and direction/cone angle;
the swirl velocity is then added.